The 2021 winter solstice occurred at 10:59 a.m. ET/7:59 a.m. PT on Tuesday, Dec. 21, marking the official start of the astronomical winter and the shortest day of the year for the Northern Hemisphere.
In the Southern Hemisphere, it's the summer solstice and the longest day of the year.
The 23.5-degree tilt in Earth's axis of rotation creates a rise and fall appearance of the sun over the course of a year. During the winter solstice, the Northern Hemisphere is tilted at its farthest distance from the sun, bringing less light and colder temperatures.
The tilt of the Earth -- not our distance from the sun -- is what causes winter and summer. In fact, Earth is closer to the sun during winter than it is in the summer months.
For those who don't like jack frost nipping at their noses, mark your calendars: The first day of spring, also called the vernal equinox, happens on Sunday, March 20, at 11:33 a.m. ET/8:33 a.m. PT.